Letter pubs.acs.org/JPCL
Ionization of Nitric Acid on Crystalline Ice: The Role of Defects and Collective Proton Movement S. Riikonen,*,† P. Parkkinen,† L. Halonen,† and R. B. Gerber†,‡,§ †
Laboratory of Physical Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014, Helsinki, Finland Institute of Chemistry and the Fritz Haber Research Center, The Hebrew University, Jerusalem 91904 Israel § Department of Chemistry, University of California Irvine, Irvine, California 92697, United States ‡
S Supporting Information *
ABSTRACT: Ionization of nitric acid (HNO3) on a model ice surface is studied using ab initio molecular dynamics at temperatures of 200 and 40 K with a surface slab model that consists of the ideal ice basal plane with locally optimized and annealed defects. Pico- and subpicosecond ionization of nitric acid can be achieved in the defect sites. Key features of the rapid ionization are (a) the efficient solvation of the polyatomic nitrate anion, by stealing hydrogen bonds from the weakened hydrogen bonds at defect sites, (b) formation of contact ion pairs to stable “presolvated” molecular species that are present at the defects, (c) rapid formation of the “solvent-separated” ion pair, which is facilitated by collective proton migration that is typical to ice, and (d) the facile formation of Eigen ions on the ice basal plane. SECTION: Environmental and Atmospheric Chemistry, Aerosol Processes, Geochemistry, and Astrochemistry
N
cirrus19 and PSCs20 (for further reading, see ref 26 and references therein). Under temperatures and nitric acid partial pressures appropriate for the upper stroposphere, two phases are typically observed:12−16,18 (1) low coverage regimepresumably nitric acid adsorbed on ice, and (2) supercooled nitric acid/water liquid. Phase 1 is characterized by a limited uptake and coverages ranging from sub- to a few monolayers, while in phase 2 a continuous and unlimited uptake is observed. Experiments that have been performed over a wide partial pressure and temperature range show some key similarities for phase 1: the low coverage13,15 of nitric acid on ice is characterized in the infrared (IR) spectra by a double peak at 1300−1500 cm −1 that corresponds to nitrate (NO 3− ) asymmetric NO stretches,27,28 a good indication that nitric acid has been ionized. A recent work studying the ionization of nitric acid on crystalline ice at 130−150 K concluded that at small coverages nitric acid is mostly molecular with increasing ionization as a function of time.29 Finally, a very recent experimental work,30 studying the ionization of nitric acid in binary water/nitric acid mixtures and on amorphous frozen water surfaces, concluded that at 45 K, within 1−2 ML coverage, nitric acid is mostly ionized as suggested by the Zundel continuum and the asymmetric NO−3 stretch band in the IR spectra, while spectral
itrogen oxides (NOx species), resulting from anthropogenic and natural sources are abundant in the atmosphere.1,2 These species are removed by oxidation, mainly as nitric acid (HNO3), which is an important sink for these reactive nitrogen molecules,2,3 while nitric acid itself is removed from the troposphere by deposition in snowpacks4 and clouds,3 contributing to acid rain and nitrate accumulation on soil.5 Nitric acid can go through “renoxification” processes, where, through heterogeneous catalysis taking place on water/ice6,7 and other surfaces, it is converted back into NOx species. Nitric acid also forms an important class of atmospherical particles, the nitrate aerosols.8 In the upper troposphere, cirrus clouds9−11 that consist of ice particles may scavenge the abundant tropospheric nitric acid concentrations efficiently,12−18 changing the amount of nitric acid in this region and also affecting the composition19 of these atmospherically relevant clouds.10,11 In the lower stratosphere, nitric acid creates nitric acid trihydrate (NAT), which forms polar stratospheric clouds (PSCs),20 an important scaffold for heterogeneous reactions releasing molecular chlorine and depleting stratospheric ozone.21 The NOx depletion and renoxification affect, through changing NOx concentrations, the amount of inert chlorine reservoirs21−23 such as ClO and ClONO2. The interaction of nitric acid with water and ice surfaces has stimulated a great deal of experimental work. Several laboratory studies have mimicked the conditions of the upper tropo-12−18 and lower stratosphere,24,25 while field experiments have measured concentrations of various chemical species both in © XXXX American Chemical Society
Received: March 8, 2013 Accepted: May 14, 2013
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Figure 1. (a) The model ice surface. The average oxygen−oxygen distance (dOO), OH bond length (dO−H), and hydrogen bond length (dH···O) with their deviations are dOO = 2.68 ± 0.07 Å, dO−H = 1.01 ± 0.02 Å, and dH···O =1.67 ± 0.09 Å. (b) In the hydrogen bonding topology of the model surface top layer, arrows denote hydrogen bond proton donation, “A” stands for proton acceptor, “D” for proton donor, i.e., “AAD” is a proton double-acceptor single-donor species, etc. (c,d) Defects created by removing either an “AAD” (c) or “ADD” (d) species. Circles stand for the site from which the molecule was removed. Dashed arrows indicate hydrogen bond formation during a local geometry optimization. The A/D naming of molecules refers to initial, unoptimized geometries.
Figure 2. Schematic illustration of an acid ionization on the model surface of Figure 1. H+ stands for proton and M− for anion (the nitrate ion in the present case). The cation site is indicated with (+). A possible rapid formation of SSIP to a surface Eigen ion site and without presolvation is depicted in panels a−e. Adsorption into a defect site and an immediate formation of a CIP is scketched in panels f−h.
In the following, we will concentrate on strictly ice-like surface structures. The key properties of our model are (1) the abundance of dangling OH bonds (present on ice surfaces,47 while scarce on ice nanoparticles44), (2) the presence of various acceptor/donor species on the surface, (3) defects,44,48 and (4) strictly ice-rule obeying molecules. The model system we employ is depicted in Figure 1a: it consists of 48 water molecules, arranged in two double layers of periodic, hexagonal ice with the basal planes of the slab facing the vacuum. The size of this periodically repeated slab is 13.2 Å × 15.3 Å. The topmost double-layer of the slab model is schematically represented in Figure 1b, where arrows denote hydrogen bond proton donation. Water molecules have been labeled according to their types, i.e., as a hydrogen bond proton double-acceptor, proton single-donor (AAD), hydrogen bond proton single-acceptor, proton double-donor (ADD), and so forth. The ice basal plane of Figure 1, featuring alternating rows of AAD and ADD, is energetically stable, and such structures are present on the basal plane at low temperaratures. For further discussion, see ref 49 and references therein. Ab initio molecular dynamics (AIMD) on the deuterated model system is performed within the density functional theory (DFT) as implemented in the CP2K/Quickstep program package.50
signatures corresponding to molecular nitric acid emerged for higher coverages.30 Nitric acid ionization has been studied theoretically by Bianco, Hynes, and Wang28,31−37 using both ab initio and mixed quantum mechanics/molecular mechanics (QM/MM) schemes and by Shamay et al.38 with ab initio simulations. These computational studies emphasize that, while nitric acid ionizes readily in the bulk or just below the surface, ionization does not take place, at least in the picosecond time scale, on the top of the aqueous film: only in a bulk-like environment may NO−3 become fully solvated by the surrounding water molecules and a suitable, 3-fold hydrogen bonded (“presolvated”) water molecule will become available that may accept the incoming proton from the nitric acid.36−38 In the present study, we explore the possibility for fast ionization of nitric acid on the top of an ice film, while in this context we emphasize the importance of defect sites and the pecularities of proton transport in ice.39,40,40 The significance of ice defects and kink sites have earlier been demonstrated in the ionization of another atmospherically appropriate species, hydrochloric acid,41−46 but, to our knowledge, this has not been discussed in detail for a molecule with a large polyatomic anion, such as nitric acid. 1851
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Figure 3. Nitric acid adsorbed on (a) the pristine ice surface, (b) surface with defect LD2, and (f) surface with defect LD1. In panels a and b, molecule species (ADD, AAD, etc.) have been indicated after (including A/D in parentheses) and before (exluding A/D in parentheses) nitric acid adsorption. The DDD indicates the Eigen ion that accommodates the extra proton. Panels b−d show various stages from a molecular dynamics run at T = 200 K. Atoms on the proton-relay path have been highlighted, while arrows indicate the periodicity of the ice model. In panels a, b, c, and f, hydrogen bond lengths from nitrate oxygens to water molecules have been indicated in Å (1 Å = 10−10 m).
“presolvated” DD (double hydrogen bond proton donor) species has been stabilized by a defect. Another interesting question is whether such defect sites can solvate the polyatomic nitrate ion. To test these mechanisms, nitric acid adsorption on the pristine surface of Figure 2a and into three different defect geometries (LD1, LD2, D2) were considered (the details of these structures are given in the Supporting Information). The local environment of nitric acid when adsorbed on a perfect ice surface (P) is depicted in Figure 3a. During an AIMD run at T = 200 K, the hydrogen bond to oxygen(III) is most of the time broken, and the N−O bond of this oxygen projects toward vacuum, with (III) forming a hydrogen bond only occasionally; the binding state of nitric acid on the pristine surface is then rather “sloppy”.38 No proton transfer is observed during a 5.8 ps simulation. On the contrary, upon nitric acid adsorption on defect LD1, a spontaneous ionization, as depicted in Figure 3e,f, occurs during a local geometry reoptimization: in Figure 3f, molecule 3 has accepted the incoming proton from the nitric acid molecule. A closer look into the hydrogen bonding of molecule 3 in Figure 2e reveals that hydrogen bonds to molecules 1 and 4 are weak, as the overall bonding of molecule 3 is far from tetrahedral. Molecule 3 can then be described approximately as a “DD” species, while the CIP formation has proceeded as suggested in Figure 2f−h. Furthermore, the nitrate ion steals one of the hydrogen bonds from molecule 3 to solvate properly. The local environment of nitric acid adsorbed into defect LD2 and optimized is illustrated in Figure 3b. During the optimization, the weak hydrogen bond between molecules 2 and 4 is stolen by nitric acid, while the defect has facilitated nitric acid to lie deeper in the surface. The species (molecule 3) that could accommodate the surface Eigen species is only one molecule away from the nitric acid proton. When AIMD run at T = 200 K is performed, the proton migration is initiated. Figure 2c shows the situation after 0.47 ps. A surface Eigen
Further details about the model and the used methodology are given in the Supporting Information. In water, oscillations of the hydrogen bonded network drive proton-relay (or “Grothuss migration”) by (a) modifying the A/D configuration of water molecules along the proton-relay path (in an otherwise ≈4-fold coordinated network) and by (b) reducing the O−O distance between neighboring water molecules.51 The same mechanisms drive the ionization of nitric acid in an aqueous environment:37 fortuitous oscillations and hydrogen bond breaking is needed to form the “contact ion pair” (CIP), while consecutive oscillations will result in further proton-relay and in the “solvent-separated ion pair” (SSIP). However, in ice, mechanisms a and b are hindered by the strictly crystalline ice structure and low temperature. On the other hand, it is known that CIPs form rapidly at defect and kink sites, at least for the diatomic hydrochloric acid,41−46 while proton migration in ice is surprisingly fast without presolvation.39,40,52 Figure 2a−e depicts a tentative formation of SSIP on the model surface of Figure 1. From Figure 2, it is clear that an optimal site for a solvent-separated proton is the surface (AAD) species, as it can transform directly into 3-fold coordinated triple hydrogen bond proton donor (DDD), which is optimal for the Eigen ion. From Figure 2 it is further observed that during the migration, the proton must pass through energetically unfavorable 4-fold coordinated cation (ADDD) sites39,40 to arrive to the SSIP. From theoretical studies of proton migration,39,40,52 it is known that in ice, (a) the Eigen form is sligthly favored over the Zundel ion and (b) proton migration becomes fast due to the unfavorable ADDD cation sites on the proton-relay path. It is then natural to ask whether the same mechanism would facilitate rapid formation of the SSIP on an ice surface, in a process illustrated by Figure 2a−e. Also, in contrast to liquid environment, ice can stabilize defects and kink sites that facilitate CIP formation. Such a mechanism is depicted in Figure 2f−h, in which we have assumed that a 1852
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process in which nitric acid creates a defect (displaces a water monomer into the vacuum) and ionizes. The ionization on the model ice surface of the present study is seen to be exothermic, while in earlier studies employing more liquid-like surfaces it was found to be endothermic. In the nitric acid/defect adsorption geometries analyzed so far, the acid was placed in an anticipated way on the defect site to test rapid CIP and SSIP formation of Figure 2. Situations where the acidic proton is slightly “off-site” from the defect were tested for LD1. Details are given in the Supporting Information. In Figure S4, nitric acid is placed nearby the defect. The binding state is “sloppy”, i.e., similar to the pristine surface case: the only hydrogen bond that is persistent throughout the 7.8 ps AIMD simulation is the bond between the acid’s proton and the water molecule of the original adsorption site. As the simulation proceeds, this water molecule becomes, upon the effect of nitric acid and sporadic proton rattling events, presolvated, i.e., a DD species, leading eventually to instanteneous CIP and SSIP. It is important to realize that this presolvation is facilitated by the neighboring defect. A case where nitric acid and the defect were further apart was tested as well, but no ionization occurred during 18 ps AIMD. Finally, a calculation was performed where the tentative ionization of Figure 2a−f was forced on the pristine surface with the aid of metadynamics. Details are given in the Supporting Information. As indicated in the inset of Figure S5, instead of a collective jump of just two, a simultaneous relay of three protons is mandatory. The state, where the collective proton relay occurs, is energetically close to the final SSIP state, while we associate most of the energetic barrier to the insufficient solvation of the nitrate ion. Nitric acid dissociation could then be stabilized even on the crystalline and “pristine” ice surface with the aid of admolecules and/or kinks providing more hydrogen bonds for the anion. To summarize, in the present work, several aspects of nitric acid ionization on ice were discussed with the aid of AIMD simulations. It was observed that (1) a polyatomic anion (the nitrate ion) can steal weakened hydrogen bonds at defect sites to become fully solvated, (2) defects (locally optimized and annealed) form stable (“presolvated”) molecular species that can directly form the CIP, and (3) the SSIP may form at low temperatures without any presolvation at all. This last mechanism is facilitated by the pecularities of proton transport in ice and by the availability of surface Eigen species on the ice basal plane. Defects also facilitated hydrogen bonded “shortcuts” to the Eigen species. Everything discussed here for defects can most likely be extended to kink sites and admolecules. For determining realistic rection rates, an important question are still the time scales that are needed for bringing nitric acid and a defect into close contact. On the other hand, in an atmospheric ice surface, defects, kinks and admolecules should be abundant. Nuclear quantum effects (NQE) may also play an important role in the kind of collective proton motion observed here. In the present case, acid dissociation by a collective proton motion was observed over two hydrogen bonds, while to achieve a jump through three hydrogen bonds (the pristine surface case), metadynamics was employed. As quantum mechanically treated hydrated protons are known to delocalize over various water molecules,53 an interesting question is whether acids can form rapidly a SSIP to the surface Eigen species on a perfect ice surface at low temperatures.
species, which is stable for 1.35 ps before further migration, is formed. Details of the migration are given in Figure 4. It is
Figure 4. Evolution of oxygen−oxygen distances (dOO) and OH (dO−H) bondlengths in a T = 200 K AIMD run, 200 fs before acid dissociation (proton-relay) for system LD2 (see Figure 3b). Proton “rattling“ (R) and jump (PT) events are indicated. Oxygen (red spheres) and hydrogen (white spheres) atoms are indicated and enumerated in the inset. Values (Δd) of distances and bond lengths are relative to the values of the initial and locally optimized structure.
observed that two protons relay almost simultaneously along the hydrogen bonded path. The formation of SSIP then proceeds without the formation of CIP (as suggested in Figure 2a−f) and is governed by a collective proton motion that is typical for ice.39,40,52 A ≈1 ps “resting” is observed on all surface Eigen sites along the proton migration path, succeeded by collective proton relays to the next Eigen site. Finally, the proton resides in a geometry depicted in Figure 2d until the end of the 7.7 ps simulation. Adsorption to a still another defect structure (D2) was tested in a similar fashion, while the LD2 case was also tested at T = 40 K and at T = 200 K with a hydrogenated (instead of deuterated) system. The results were practically identical to the deuterated T = 200 K case. According to Figure 5, the
Figure 5. Evolution of oxygen−oxygen distances (dOO) and OH (dO−H) bondlengths in a T = 40 K AIMD run, 200 fs before acid dissociation (proton-relay) for system LD2 (see Figure 3b). A proton jump (PT) event is indicated. For more details, see the insets and caption of Figure 4.
deuterated low-temperature T = 40 K case is particularly interesting: the system is undisturbed by temperature fluctuations and proton “rattling” events, yet the collective proton relay proceeds effectively. In Table S1 of the Supporting Information, a few reaction energies, based on our calculations, are given: the closest energetic match to the available experimental data is found for a 1853
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ASSOCIATED CONTENT
S Supporting Information *
Details of ice surface model and computational methods, details of energetics and comparison to earlier results in the literature, and complementary and detailed results of surface defect structures and proton movement. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: sampsa.riikonen@iki.fi. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Finland’s Center for Scientific Computing (CSC) for the use of its computational resources. All authors acknowledge support by the Academy of Finland through the FiDiPro and Lastu programs, while R.B.G. additionally acknowledges the Israel Science Foundation (Grant 172/12) and the U.S. National Science Foundation (Grant 0909227).
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